1. Field of the Invention
The present invention pertains to apparatus for use with internal combustion engines to reduce emission of noxious gases and noise emitted therefrom. More specifically, the present invention pertains to apparatus, particularly suited for use with spark ignited, four cycle, carbureted or injected engines, for converting nitrogen oxides, carbon monoxides and unburned hydrocarbons from the exhaust of such engines to less noxious compounds and for reducing the noise emitted therefrom.
2. Brief Description of the Prior Art
Both noise and air pollution have been of increasing concern in recent years. Silencers or mufflers for noise reduction of internal combustion engines have existed for many years. Most silencers utilize some type of housing in which is mounted various types of baffles or other silencing components for reducing the noise produced at the exhaust of an internal combustion engine.
As early as 1957, automotive and industrial catalytic converters were being developed to reduce carbon monoxide and unburned hydrocarbons emitted from internal combustion engines. In these early designs, beads or pellets of catalyst were contained in a basket or cage to which exhaust gases were directed for radial flow (inwardly or outwardly). Such designs resulted in increased pressure drop, required frequent replacement of catalyst and required inspiration of atmospheric air to aid in oxidation of carbon monoxide and unburned hydrocarbons. Such a design is shown in U.S. Pat. No. 3,899,303.
In later emission designs, catalytic converters were developed in which a catalyst formulation was deposited on a monolithic substrate of extruded ceramic or metal corrugations to form a honeycomb type monolithic catalyst. Such a design is shown in U.S. Pat. No. 4,579,194. However, like the design of previously mentioned U.S. Pat. No. 3,899,303 this design also required inspiration of atmospheric air to aid in oxidation of carbon monoxide and unburned hydrocarbons. In such designs, exhaust gas flow can be interrupted or reversed through aspiration inlets should back pressure increase in the catalyst due to collection of particulates, backfires, etc. This would result in directing untreated, polluted exhaust gases to the atmosphere.
In early years of catalytic converter development, only carbon monoxide and unburned hydrocarbons were identified as smog producing pollutants. The catalyst used in early two-way catalytic converters, such as those heretofore mentioned, was a platinum formulation which required additional oxygen to support catalytic reaction. Thus these designs (U.S. Pat. Nos. 3,899,303 and 4,579,194) required induction of air.
There is now even more heightened concern for air pollution created by noxious gases emitted from the exhaust of an internal combustion engine, primarily carbon monoxides, other unburned hydrocarbons and more recently recognized nitrogen oxides. Catalytic converters have been developed through which the exhaust gases may be passed for converting the nitrogen oxides, carbon monoxide and unburned hydrocarbons to less noxious compounds before being discharged to the atmosphere. Such catalytic reactions require close control of air-fuel ratio and three-way catalytic converters using, for example, a formulation of platinum, palladium and rhodium. No outside introduction of air or oxygen is required or desired. Problems associated with three-way catalytic converters have been identified as: imprecise air/fuel ratio control, deactivation of the catalyst and flow maldistribution at the front area of the catalytic converter. The flow distribution problem is the one least improved and the one which holds most promise in increased efficiency and extended catalyst life.
Exhaust silencers and catalytic converters for internal combustion engines are governed by chemical, fluid flow, and acoustical characteristics. These characteristics interact with each other and when properly combined provide an efficient exhaust system for control of air and noise pollution.
Air pollution and noise abatement equipment may take many forms or configurations. For example, the engine may be provided only with a catalytic converter or with a catalytic converter first and then a silencer or with a silencer first and then a catalytic converter. In more recent years, catalytic converters and exhaust silencers have been combined in a single housing. See, for example, U.S. Pat. Nos. 5,016,438 and 5,921,079.
Especially in the manufacture of a combined catalytic converter and exhaust silencer, all three disciplines or characteristics (chemical, fluid flow and acoustical) must be considered and properly balanced. Exhaust silencing quality, emissions reduction efficiency, and engine efficiency all depend upon a proper combination.
As previously stated, one of the primary problems to consider in a combined catalytic converter and exhaust silencer, particularly for efficiency of catalytic conversion, is flow distribution. In recent years, research has been conducted to determine the velocity profile of exhaust gases entering the frontal area of the catalyst module of a catalytic converter. One such study has been reported in a paper entitled Improvement of Catalytic Converters for Stationary Gas Engines by Using a Metal-Supported Catalyst, Y. Tsurumachi and A. Fujiwara and Y. Yamada all of Tokyo Gas Co., Ltd. Tokyo, Japan. Another study is reported in Chemical Engineering Science, Vol. 44, No. 9, pp. 2075-2086, 1989 in an article entitled Transient Operation of Monolith Catalytic Converters: a Two-Dimensional Reactor Model and the Effects of Radially Nonuniform Flow Distributions. Research has produced evidence that exhaust gases entering a catalytic converter housing at high velocity concentrate in the center core of the catalytic converter module in a pattern substantially the same diameter and area as the inlet. Other results of research show that when a truncated conical inlet transition is provided exhaust gases recirculate around the transition causing flow maldistribution and scattered hot and cold spots in the frontal area of the catalyst module, reducing conversion efficiency.
Catalytic converters of relatively small frontal area result in increased impingement velocity and higher linear velocity through the catalyst depth. This creates higher back pressures, raising the mean effective pressure in the exhaust system between the catalytic converter and the engine exhaust valves. Undersized exhaust silencers can also increase the back pressure and when coupled with an undersized catalytic converter multiply the back pressure many times over, reducing operational efficiency of the engine.
Velocity control and pulsation dampening are products of capacity made possible by diameter and length. Diameter controls velocity and length controls expansion of exhaust gases to achieve design velocity and reduced pulsation level. In addition, staging pulsation (manifested as vibration and noise due to engine firing frequency) helps flow condition the exhaust gases upstream of the catalyst to assure even distribution of exhaust gases to the catalyst face.
From a chemical standpoint, residence time of exhaust gases through a catalyst module is important for catalytic reduction of toxic emissions. Research has determined that the linear velocity through the catalyst should be between 14 and 17 actual feet per second to provide optimum back pressure and residence time for conversion. In order to efficiently utilize the three characteristics or disciplines in the design of an internal combustion engine exhaust system, the catalytic converter module should be manufactured to accept exhaust gases at the prescribed velocity, maintaining linear velocity through the catalyst module depth while providing optimum residence and conversion therethrough.
Catalyst size and substrate material in proportion to mass flow and back pressure limits is also important in automobile applications of catalytic converters. Physical manufacture in keeping with good limits of mass production methods in the automotive muffler industry is also important.